This invention relates to a rotary piston machine for compressible media with at least two rotary pistons sealed in a common housing and rotatable in a controlled manner with one another, the rotary pistons having a plurality of disk-shaped sections engaging in one another by pairs and whose thickness reduces in the direction of the pressure side, each disk having at least one surface area and one core area formed by directrices along arcs of circles with centre on the axis of the respective rotary piston and connected by an interface area respectively.
Rotary pistons for vacuum pumps or displacement pumps for gases are usually manufactured in the form of screw spindle pairs. For the purpose of displacement or compression these screw spindles have a variable pitch. Screw compressors for gases with two screws engaging in each other and whose pitch reduces constantly towards the pressure side are known. Although such compressors enable high compression ratios to be achieved, the manufacture of screw spindle pairs with variable pitch axes is technically difficult, especially as the screws should engage in each other free from play as far as possible in order to keep pressure losses low. This means that the manufacture of this type of screw compressor is expensive.
On the other hand so-called Roots blowers are known with two disk-shaped rotary pistons engaged in one another. The air throughput occurs diametrically opposed to the rotation axes of the rotary pistons, so that such compressors are suitable for large quantities of air but for low compression ratios only. In order to achieve higher compression ratios several compressor units of this type have to be connected in series, or assembled to form a multi-stage Roots pump.
In order to avoid the difficult manufacture of screw spindles with variable pitch, the suggestion has already been made to develop the rotary pistons as diminishing-step rotary pistons.
DE-2934065 discloses such diminishing-step rotary pistons in a rotary piston machine of the type mentioned at the start of the text. In this machine the spindles have a pseudo-thread-like groove formed by graduated recesses provided with peripheries at right angles to the spindle axes and following one another in the screw line. In this groove engages, in the plane delineated by the two spindle axes, a correspondingly formed thread-type comb in the counter spindle and delineates a groove volume with each turn, so that as the spindles roll off one another the comb displaces the groove volumes with compressible medium from inlet to outlet, the groove volumes changing and the desired pressure difference between inlet and outlet being achieved. In their cross-sections the spindles have a semi-circular contour with a cutout delineated by the core area and two step-forming interface areas. The sector angles of the external surface areas and inner core areas have the same value, namely 180xc2x0. The disadvantage with this rotary piston machine is the large number of step-shaped peripheries which are necessary in order to form the pseudo-thread-like groove, whose manufacture requires a large number of machining processes. A further disadvantage is the high degree of interface precision required to minimise pressure losses from stage to stage.
A simplified construction of diminishing-step rotary piston is disclosed in DE-2944714. This prior printed publication suggests a laminated construction of rotary pistons with each rotor comprising a plurality of single disks with identical face profile, namely with surface areas and core areas with 180xc2x0 sector angles each, but with varying thicknesses or diameter. The absent sealing effect between rotary pistons of this construction, which creates gas backflow and a low compression ratio, ought to be compensated for by high-speed operation, but this in turn creates thermal and mechanical problems as well as high noise levels.
The prior printed publication AT-261792 also describes a rotary piston machine of this type in which the diminishing-step rotary pistons comprise single disks with identical cross sections. Each disk has two external surface areas diametrically opposed to one another and two internal core areas diametrically opposed to each other whose sector angles are all the same (90xc2x0). With this design of disk and this offset arrangement in the rotor the gap widths between opposing disks must be kept as low as possible. The surface and core areas are therefore connected by interfaces developed as extended epicycloids in order to create the sealing effect between the disks. Consequently both their profile and the external synchronising device of the machine must be very preciselyxe2x80x94and therefore expensivelyxe2x80x94manufactured. Although this prior printed publication provides for the reduction of the thermal loading of the edge tips by means of a rounded shape, these cannot be avoided with gas backflow.
This invention relates to the manufacture of a rotary piston machine with high compression ratio, in particular of a vacuum pump, in which the end vacuum is designed to be better than with rotary vane pumps, approximately similar to that of multi-stage Roots pumps. In doing so, manufacture should be less costly than that of multi-stage pumps and also less expensive than that of screw pumps. Furthermore, internal compression of the compressible medium or gas is meant to occur in order to achieve a reduction in energy consumption and operating temperature. Finally, noise levels during operation should be as low as possible.
These objects are achieved in a rotary piston machine of the type initially cited, in which the sector angles of the surface area and of the core area of a respective disk are not identical, the disks have various transverse profile contours periodically recurrent along the piston shaft, and each disk is offset at an angle to the two adjacent disks of the same rotary piston in such a way that these three disks have a common directrix via one section of their core areas and form a chamber.
With this type of construction a graduated spiral pitch with horizontal intermediate sections between two chambers is formed in the individual non-assembled rotary piston. A chamber sequence is formed in the axial direction with selectably variable volume, i.e. selectably variable internal compression through selectable thickness variation on the disk-shaped sections.
The use of sequences of disk-shaped sections of various transverse profile contours means that, with a specified number of chambers, the overall number of sections can be kept lower than is the case with the rotary piston machines with state-of-the-art diminishing-step pistons.
With a low number of sections each rotary piston can be manufactured in one piece which substantially improves dimensional stability and is less thermally critical than a stack of single disks. If the operating temperature of the rotary piston machine is low due to the way it is used, the rotary pistons can also be made up of sequences of single profile disks arranged axially one on top of the other, which saves manufacturing costs.
In the following specification the word xe2x80x9cdiskxe2x80x9d, unless otherwise specified, is used for both individual profile disks as well as disk-shaped sections of a one-piece piston.
The displacement machine according to the invention is contactless and constantly rotating. The gaps between the two rotary pistons rotating with one another can be sub-divided into three types
a. Surface area/core area of opposing disk-shaped sections: these linear gaps are determined by the precision of manufacture of the cylindrical areas of the pistons and the distance between the two rotating axes. Low gap values can be achieved with current manufacturing technology.
b. Frontal area/frontal area of disk-shaped sections lying one on top of the other: the gap widths of these flat gaps can also be kept low using modern production machines. The large gap lengths, along the direction of flow between the rotary pistons, effect a good seal and therefore a good end vacuum.
c. Interface area/interface area of opposing sections, in particular tips/concave flank: with the offsetting according to the invention of the disk-shaped sections these gap widths are not critical and can lie within the millimetre range which facilitates substantially the manufacture of the interfaces. As these gap widths also determine the permissible angle play between the rotary pistons, this permissible angle play is very large, which means that the requirements for the synchronising device of the rotary piston machine are reduced and their selection or realisation rendered simpler.
The theoretical cycloid-shaped curved interface areas, i.e. the parallelepiped areas which connect respectively the surface area and the core area, i.e. external cylinder and core cylinder, of a disk-shaped profile section, when the rotary pistons are rotating in the opposite direction, do not have any critical sealing function essential to operation and therefore describe a theoretical maximum contour. A profile contour of the interface area can be made somewhat smaller or flatter than this theoretical maximum contour and manufactured more easily, for instance a contour without undercut and/or virtually straight, and can therefore be preferred and is very efficient in operation. The permissible angle play in operation is also increased as a result.
For practical purposes both adjacent disks of a disk with an external surface area, whose sector angle is greater than the sector angle of the core area, have external surface areas whose sector angles are less than the sector angles of the core areas.
For practical purposes the difference between the sector angles of the external surface area and of the core area of a disk-shaped section is large. The sector angle of this surface area, for a disk with a small external surface area, is preferably less than 90xc2x0 and, more preferably still, less than 60xc2x0. Such a disk opposes a disk of the other rotary piston, with a sector angle of the external surface area correspondingly larger than 270xc2x0 respectively greater than 300xc2x0.
The chambers of a respective rotary piston are preferably designed in such a way that the interface areas of a disk form, respectively with an interface area of an adjacent disk, a continuous interface area with common directrix.
The synchronising device of the rotary piston machine according to the invention can be selected in such a way that the two extra-axial rotary pistons have a contrarotating direction of rotation. The outer diameters of the rotary pistons, the diameters of the core cylinders and the translation can then be selected in such a way that the pistons roll off each other without sliding, the surface area of a disk-shaped section rolling off the core area of the opposing section. If the number of surface areas and core areas of a disk-shaped section are respectively identical to those of the opposing section of the other rotary piston, then a translation of 1:1 is to be selected. If these numbers vary, however, then the translation must be selected accordingly.
In other embodiments with asymmetrical energy distribution the two extra-axial rotary pistons have the same direction of rotation.
In still other, compact embodiments the two rotary pistons are intra-axial, i.e. are designed as external rotor and internal rotor with an additional G rotor.
In several rotary piston designs the disk-shaped sections of a respective rotary piston have only two alternating face section profile contours.
Moreover, the diameters of the surface/outer cylinders and the core cylinders of extra-axial rotary pistons may be respectively identical, the section of the first piston having the one face section profile contour, whilst the opposing section of the second piston has the other face section profile contour, in a same plane at right angles to the piston axis.
The two rotary pistons can also be designed as main rotor and auxiliary rotor with different diameters and therefore varying shaft outputsxe2x80x94up to 100:0%xe2x80x94which is advantageous for the execution of the synchronising device.
In some such embodiments of the rotary pistons, sequences of sections with various face cut profile contours alternate with circular locking disks, so that a respective piston has sections with three or more different profile contours.
Further features and advantages of the invention will emerge for the person skilled in the art from the description which follows of several preferred embodiments and from the accompanying drawings.